whenComplete method

The action function is called when this future completes, whether it
does so with a value or with an error.

This is the asynchronous equivalent of a "finally" block.

The future returned by this call, f, will complete the same way
as this future unless an error occurs in the action call, or in
a Future returned by the action call. If the call to action
does not return a future, its return value is ignored.

If the call to action throws, then f is completed with the
thrown error.

If the call to action returns a Future, f2, then completion of
f is delayed until f2 completes. If f2 completes with
an error, that will be the result of f too. The value of f2 is always
ignored.